We don't really know. Thousands of new letterboxes are planted around the world every month, and there is no single source that catalogs them all. Atlas Quest and Letterboxing North America (LbNA) are the two biggest online sources of clues, but thousands of clues are passed around as Word-of-Mouth (WOM) clues, are listed on personal websites, and have alternative methods of distribution. If you must pin down a number, be vague. Say something like "there are in excess of 25,000" letterboxes in the United States. If you want precise numbers, you can check out the Statistics Page here at Atlas Quest, but please note that they only refer to letterboxes listed on Atlas Quest. Thousands and thousands more exist.

Do not use photos of stamps, letterboxes, or clues without permission of the box's owner. We put a lot of effort and work into our letterboxes and are very protective of them and how they're presented in the media. Usually, it's not hard to find someone more than willing to share their creations, but it's considered bad form to publish photos and clues of another person's letterbox without permission.

Contacting people who've planted letterboxes in the area you cover is your best option, but you can also ask for people who might be interested on an appropriate message board on Atlas Quest. In most cases, posting to the state where your readers or viewers are located would be the best board for posting such requests.